Start on adding support for editing via the assistant panel (#14795)

Note that this shouldn't have any visible user-facing behavior yet. The
feature is incomplete but we wanna merge early to avoid a long-running
branch.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -3639,6 +3639,12 @@ impl MultiBufferSnapshot {
                         text: item.text,
                         highlight_ranges: item.highlight_ranges,
                         name_ranges: item.name_ranges,
+                        body_range: item.body_range.and_then(|body_range| {
+                            Some(
+                                self.anchor_in_excerpt(*excerpt_id, body_range.start)?
+                                    ..self.anchor_in_excerpt(*excerpt_id, body_range.end)?,
+                            )
+                        }),
                     })
                 })
                 .collect(),
@@ -3668,6 +3674,12 @@ impl MultiBufferSnapshot {
                         text: item.text,
                         highlight_ranges: item.highlight_ranges,
                         name_ranges: item.name_ranges,
+                        body_range: item.body_range.and_then(|body_range| {
+                            Some(
+                                self.anchor_in_excerpt(excerpt_id, body_range.start)?
+                                    ..self.anchor_in_excerpt(excerpt_id, body_range.end)?,
+                            )
+                        }),
                     })
                 })
                 .collect(),